When saying type of fly to use - are you meaning what what category of fly to use when or the nuts n bolts of what the fly is made of in that there's a million variations of the same fly pattern so what makes this one better than that?

Either way, I think it boils down to being confident in your pattern regardless the water. The confidence being the culmination of combining the understanding of what to use when and how to use it. Pattern the fish first - make an objective judgement on what the fish are doing in order to select a category of fly (i.e. nymph, emerger, dry) then narrow down size and color if you can.... then make your subjective choice of what fly you actually tie on - what you like that can fit the situation ....sort of generally "matching the hatch" but more like "close enough for government". The actual fly you end up tying that can best resemble those criteria is normally a personal preference. Example, I may notice that fish are active at the surface and see that they are taking surface emergers - I may throw a greased leader generic soft hackle with traditional materials with similar color n size, but my buddy may fling his his flush riding, cut wing, biot bodied, microfibbeted, pantoned color matched corvette of a fly - and we both have a great day of catching....
Not that this is a debate on this thread - I'd have to be neutral on pattern over presentation or presentation over pattern....- It's like light and dark -you can't have the one with out the other...so I think it's definately a combo and that combo depends on the water your on as to what is priority - ultimately, though if you don't have any confidence in your fly, it won't stay in the water long enough to catch fish.